Extreme altitude starts at 5,500m (18,000ft), where the effective oxygen level is half that of sea level. That's the height of Kala Patthar - the highest point on the Everest Base Camp trek - and just below Kilimanjaro at 5,895m tall.
There are five big things that happen to your body at this altitude.
1. Reduced Cognitive Abilities
Studies have shown that your brain power starts to decrease slightly at around 5,000m and then decreases rapidly above 8,000m (the 'death zone'). In his book Into Thin Air, Jon Krakauer even describes not recognising team mates during his summit of Everest (8,848m).
Of course, gaining height rapidly can also lead to more serious complications such as HAPE or HACE which can be deadly if not treated. Typically the best treatment for altitude sickness is rapid descent to lower altitudes, to recover.
2. Lower Athletic Performance
At high altitudes, humans can actually move faster for very short distances, as there is less air resistance and the body is typically working anaerobically.
At extreme altitudes, however, this effect is lost and the altitude has a marked impact on your ability to move fast. Climbers often describe forcing themselves to place on foot forward every 5 seconds at extreme altitude!
3. Your heart beats faster
Your body needs more Oxygen, so the heart tries to deliver it! Of course, there are limitations to this which leads us to..
4. You produce more red blood cells
The human body really is incredible at altitude. More red blood cells are produced rapidly (particularly in the first few days of reaching a higher altitude) so that your body can receive more oxygen. This slows after the first few days but full acclimatisation to Extreme altitudes (the point at which your body has produced its maximum number of red blood cells) can take around two months.
5. Loss of appetite
This one doesn't affect everyone, but most climbers of mountains such as Everest complain of a loss of appetite. Experienced mountaineers often recommend bringing simple foods that are easy to digest, but even still, Everest climbers typically loose around 7kg (15lb) during the month of their ascent.